Synchronized driving waveforms for four-particle electrophoretic displays

Abstract

The present invention provides improved driving methods for four particle electrophoretic displays. The driving methods improve the color state performance when a first pixel is displaying a mixed state of a first highly-charged particle and a second lower-charged particle of the opposite polarity, while a neighboring pixel is displaying a state of a second highly-charged particle having the opposite polarity to the first highly-charged particle. The particles can be, for example, all reflective or one type of particle can be partially light transmissive.

The invention claimed is: 
     
       1. A method of driving a display layer disposed between a viewing surface including a light-transmissive electrode and a second surface on the opposed side of the display layer from the viewing surface, the second surface including a first driving electrode and a second driving electrode, the display layer including an electrophoretic medium comprising a fluid and first, second, third and fourth types of particles dispersed in the fluid,
 wherein the first, second, third and fourth types of particles have respectively first, second, third, and fourth optical characteristics differing from one another, the first and third types of particles having charges of a first polarity and the second and fourth types of particles having charges of a second polarity, opposite the first polarity, and the first and third types of particles do not have the same charge magnitudes, and the second and fourth types of particles do not have the same charge magnitudes, 
 the method comprising the following steps: 
 (i) applying a first electric field having a high magnitude and the first polarity to the first drive electrode for a first time period to drive the first type of particles towards the viewing surface, thereby causing the display layer to display the first optical characteristic at the viewing surface above the first drive electrode; 
 (ii) applying a second electric field having the high magnitude and the second polarity to the second drive electrode for a second time period to drive the second type of particles toward the viewing surface, thereby causing the display layer to display the second optical characteristic at the viewing surface above the second drive electrode, wherein the first electric field is applied to the first drive electrode simultaneously while the second electric field is applied to the second drive electrode; 
 (iii) applying no electric field to the first drive electrode for a third period following the first period; 
 (iv) applying a third electric field having a low magnitude and the first polarity to the second drive electrode for a fourth time period overlapping with the third time period to drive the third type of particles toward the viewing surface, thereby causing the display layer to display the third optical characteristic at the viewing surface above the second drive electrode; 
 (v) repeating steps (i)-(iv); and 
 (vi) applying a fourth electric field having the same magnitude and polarity as step (ii) to the second drive electrode for a fifth time period, thereby causing the display layer to display a mixture of the second and third optical characteristics at the viewing surface above the second drive electrode while displaying the first optical characteristic at the viewing surface above the first drive electrode. 
 
     
     
       2. The method of  claim 1 , wherein the first time period is shorter than the third time period. 
     
     
       3. The method of  claim 2 , wherein the second time period is shorter than the fourth time period. 
     
     
       4. The method of  claim 3 , wherein the fifth time period is longer than the first time period and shorter than the third time period. 
     
     
       5. The method of  claim 1 , wherein the magnitude of the third electric field is less than 50 percent of the magnitude of the second electric field. 
     
     
       6. The method of  claim 1  wherein the first time period and the fourth time period do not overlap in time. 
     
     
       7. The method of  claim 1 , wherein the first and third types of particles are negatively charged and the second and fourth types of particles are positively charged. 
     
     
       8. The method of  claim 7 , wherein the first type of particle is yellow, the second type of particle is black, the third type of particle is white, and the fourth type of particle is red. 
     
     
       9. The method of  claim 1 , wherein the first type of particle is light transmissive, the second type of particle is yellow, the fourth type of particle is white, and the first and third types of particles are red and blue (in either order).